The invention relates to a process for manufacturing ceramic, diffusion-limiting layers using finely dispersed, ceramic powder, as well as to use of the layer.
A porous ZrO.sub.2 ceramic is known, for example, from German published patent application DE 29 45 020 A1, whose porosity or gas permeability is regulated by the addition of TiO.sub.2 to ZrO.sub.2. The porosity increases with increasing TiO.sub.2 content. For establishing the porosity, a briquette made of a powder mixture of ZrO.sub.2 (fully stabilized) and TiO.sub.2 is produced which, following sintering at 1470.degree. C., has a gas permeability of about 10.sup.-5 to 10.sup.-3 bar ml/sec cm.sup.2. It is further disclosed in DE 29 45 020 that a porous molded element, for example as a platelet, is mounted on an electrode of an electrochemical measuring sensor and consequently serves as a porous protective layer. For fastening such a platelet on a measuring sensor a glazing is suggested.
Furthermore, German published patent application DE 42 31 966 A1 describes a porous ceramic layer as a diffusion barrier or as a diffusion channel. Here as well, the layer consists of ZrO.sub.2, wherein the porosity of the diffusion layer is created by the addition of pore-forming materials, as for example thermal carbon black, theobromine or indanthrene blue, which burn, decompose or vaporize during the sintering process. The amounts of such pore-forming materials added in accordance with DE 42 31 966 is relatively high, so that a porosity of 10 to 50% is created with a mean pore diameter between 5 and 50 .mu.m. The ZrO.sub.2 powder and the pore-forming materials are brought into contact with organic binding agents and solvents, so that a ceramic green sheet can be produced from this, which is processed further in the socalled tape technology. Glass carbon is also mentioned in German published patent application DE 43 43 315 A1 as an additional pore-forming material, which is here further processed, likewise with a ceramic powder, into a tape or a paste. Depending on the grain diameter of the glass carbon, pore sizes between 1 and 150 .mu.m are attained following the sintering of the material according to DE 43 43 315.
The disadvantages of these possibilities of manufacturing porous ceramic layers according to the cited prior art lie in that the pore or pore channel diameters are relatively large for the use as gas permeable layers in gas sensors, whereby the danger exists of becoming stopped up by particles from the combustion gas stream, with the consequence of a short lifetime of sensors of this type. Moreover, for tube-shaped sensors the tape technology disclosed in DE 42 31 966 and DE 43 43 315 is not especially suited, since the usual lamination is more suited for planar and not for spherical surfaces.